I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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Alternatives October 2005 • Shallow groundwater (60 feet below ground surface) would require dewatering (removal of shallow groundwater) during construction and regular dewatering of the underpass after construction could be required. • To convey stormwater runoff collected in the underpass to the Santa Cruz River, pumping facilities would be required, which would result in higher personnel and maintenance costs. • The underpass would require 24-hour roadway lighting for security, traffic safety, and to accommodate pedestrian and bicycle traffic. • To accommodate the eventual shift in the I-10 mainline alignment recommended in the I-10 General Plan, an I-10 bridge over Twin Peaks Road would need to be excessively wide. To accommodate the future elevation increase proposed for the I-10 mainline in the I-10 General Plan, reconstruction of the I-10 mainline to match the change in elevation at the bridge would be required. The excess bridge width and reconstruction of the I-10 mainline would add considerable costs to the Twin Peaks Road TI. Advantages of the underpass option included: • Relocation of overhead electric transmission lines and support structures would not be required. • Twin Peaks Road would pass under I-10; therefore, there would be less visual effects to users of I-10. In addition, an underpass would be less visible, although the bridge over the Santa Cruz River would still be visible. • Less traffic noise would be expected to emanate from a depressed structure than an elevated structure; therefore, traffic noise mitigation could be less extensive. Alternative TI Design Options - Twin Peaks Road Over I-10 and UPRR Disadvantages of the overpass option included: Advantages: • Train traffic could be disrupted during placement of the bridge deck and possibly other phases of railroad bridge construction. • The overpass structures would be visible from I-10 and to area residents. • More traffic noise would be expected to emanate from an elevated structure than a depressed structure; therefore, traffic noise mitigation could be more extensive. • Less disruption to train traffic would be expected than with the underpass alternative. • Users of the bridge would experience a greater range of views. • The bridge structure would offer the opportunity for public art treatments visible from I-10 and at greater distances. Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 3-5 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
Alternatives October 2005 • Relocation of underground utilities between the railroad and I-10 mainline would not be extensive. • Pumping facilities would not be required to convey stormwater runoff to the Santa Cruz River. • Less extensive roadway lighting would be required. • Construction of a bridge would not be expected to involve extensive dewatering of groundwater during and after construction. • Eventual I-10 mainline realignment could be accommodated by bridge design without reconstruction of the existing I-10 mainline. The disadvantages of the underpass design option were considered much more substantial than those of the overpass design option; therefore, the overpass option was selected as the preferred design option. As a result, the design option of Twin Peaks Road crossing under both I-10 and the UPRR was eliminated from further consideration. Alternative TI Alignments After selecting a tight diamond interchange with a Twin Peaks Road overpass of I-10 and the UPRR, alternative alignments for the overpass were developed. The range of alignment alternatives was defined and each alignment alternative was compared based on design characteristics, social, economic and environmental affects, cost, and constructability. The range of alternative alignments was framed by required or desirable design characteristics. For this project, a set of design criteria was established for the I-10 mainline, the Twin Peaks Road TI and ramps, and approaching roadways (Twin Peaks Road, Linda Vista Boulevard, and El Camino de Mañana). Roadway design criteria contained in the ADOT Roadway Design Guidelines were used for the design of freeways, frontage roads, ramps, cross roads, and other roadway infrastructure within ADOT right-of-way. All other roadways used design criteria contained in the Pima County Roadway Design Manual. The north and south alignment limits were established by applying the Twin Peaks Road design speed of 50 mph and the corresponding maximum allowable roadway curvature; therefore, three alternative alignment alternatives were evaluated: north, center, and south alignments. These alignments are presented in Figures 3-4A-C. The roadway design was complicated by the proximity of the UPRR tracks to the 138 kilovolt transmission line towers east of I-10 (approximately 150 feet apart), which forced the design to “thread the needle” between the two structures (avoiding the relocation of the transmission line towers was considered desireable). Railroad safety standards require that the bottom of the bridge crossing the UPRR be a minimum of 23 feet above the railroad track surface and the Electric Safety Code requires a minimum of 22 feet of clearance between the overhead transmission lines and the roadway to provide safe vehicle passage below the lines. Because the transmission lines sag between towers, alignments were located close to the towers to meet the required vertical clearance. Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 3-6 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
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Page 18 and 19: Introduction October 2005 CHAPTER 1
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Conclusion October 2005 Interstate
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Bibliography October 2005 CHAPTER 8
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Bibliography October 2005 Tetra Tec
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I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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